The focal zone of the electrohydraulic generator is much larger than the piezo-electric one, why?

For the same aperture the focal zone of the electrohydraulic generator is much larger than the piezoelectric one. In a previous work we shown that the focal zone obtained with the same generator but where the inter space of the electrodes is filled with an electrolyte instead of water was 100 times lower and approximatively equal to the piezoelectric focal zone. So we think that the conventional electrohydraulic generator does not work well. Some measurements made using ombroscopy, film and triangulation confirm that the plasma formation takes place not always exactly at the first foci. After measuring the pressure in function of the off axis we conclude that the plasma formation is responsible of the large focal zone of the conventional electrohydraulic generator.